An image sensor is a semiconductor device for converting optical images into electric signals, and is mainly classified into a charge coupled device (CCD) and a complementary metal oxide silicon (CMOS) image sensor.
The CCD has a plurality of photodiodes (PDs), which are arranged in the form of a matrix in order to convert optical signals into electric signals. The CCD includes a plurality of vertical charge coupled devices (VCCDs) provided between photodiodes vertically arranged in the matrix so as to transmit electric charges in the vertical direction when the electric charges are generated from each photodiode, a plurality of horizontal charge coupled devices (HCCDs) for transmitting the electric charges that have been transmitted from the VCCDs in the horizontal direction, and a sense amplifier for outputting electric signals by sensing the electric charges being transmitted in the horizontal direction.
However, such a CCD has various disadvantages, such as a complicated drive mode, high power consumption, and so forth. Also, the CDD generally requires multi-step photolithography processes, so the manufacturing process for the CCD may be relatively complicated.
In addition, since it is difficult to integrate a controller, a signal processor, and an analog/digital converter (A/D converter) onto a single chip of the CCD, the CCD may be unsuitable for compact-size products.
Recently, the CMOS image sensor is spotlighted as a next-generation image sensor capable of solving certain problems of the CCD. The CMOS image sensor is a device employing a switching mode to sequentially detect an output of each unit pixel by MOS transistors, in which the MOS transistors are formed on a semiconductor substrate corresponding to the unit pixels through a CMOS technology and which may use peripheral devices, such as a controller and a signal processor. That is, the CMOS sensor includes a PD and MOS transistors in each unit pixel, and sequentially detects the electric signals of each unit pixel through the MOS transistor in a switching mode to realize an image.
Since the CMOS image sensor makes use of CMOS technology, the CMOS image sensor has advantages such as low power consumption and a relatively simple manufacturing process with a relatively smaller number of photolithographic processing steps. In addition, the CMOS image sensor allows the product to have a compact size, because the controller, the signal processor, and the A/D converter can be integrated onto a single chip of the CMOS image sensor. Therefore, the CMOS image sensor has been extensively used in various applications, such as digital still cameras, digital video cameras, and so forth.
Meanwhile, the CMOS image sensors are classified into 3T, 4T and 5T-type CMOS image sensors according to the number of transistors per unit pixel. The 3T-type CMOS image sensor includes one photodiode and three transistors per unit pixel, and the 4T-type CMOS image sensor includes one photodiode and four transistors per unit pixel.
Hereinafter, details will be described regarding the layout for the unit pixel of the 4T-type CMOS image sensor.
FIG. 1 is a layout representing the unit pixel of the related 4T-type CMOS image sensor.
As shown in FIG. 1, an active area is defined in a unit pixel (10) of the CMOS image sensor, and an isolation layer is formed on a semiconductor substrate except for the active area. One photodiode (PD) 16 is formed in a wide portion of the active area, and gate electrodes 13, 14, 20, and 30 of four transistors overlap with remaining portions of the active area.
A transfer transistor incorporates the gate electrode 13, and a reset transistor incorporates the gate electrode 14. A drive transistor incorporates the gate electrode 20, and a select transistor incorporates the gate electrode 30.
The unit pixel 10 includes a floating source/drain impurity area 18 formed on a surface of the semiconductor substrate between the gate electrodes 13 and 14 in the active area of the semiconductor substrate.